Synthesis, Characterization, And Magnetic Properties Of Barium Hexaferrites With Excessive Iron

How to improve the saturation magnetization and lower the coercivity is an urgent topic for the application of M-type barium hexaferrite (BaM) materials to the microwave signal processing. However, the available research methods of the ion substitution technique and traditional heat treatment process have been intensively studied, are facing some difficulties that can not be overcome. So it is absolutely necessary to employ new idea of material design and innovative preparation technique. The controlled process of synthesizing the BaM materials with excessive iron (Fe³⁺ or Fe²⁺), high saturation magnetizations and low coercivities by spark plasma sintering (SPS) technique was investigated in this paper. The occupation and the effects of excessive Fe³⁺ ion on the structure and magnetic properties have been comprehensively studied.M-type barium hexaferrites with excessive Fe³⁺ and Fe²⁺ were first designed with the chemical formulas of BaFe_12+xO_19+1.5x and BaFe_12-x³⁺Fe_1.5x²⁺xO₁₉(0≤x≤0.4). The precursor ultrafine powders in the step ofΔx = 0.1 were prepared by coprecipitation method. The phase composition, microstructure, and crystalline reaction mechanism of the SPS sinters of the precursor ultrafine powders of BaFe_12.4O_19.6 and BaFe_11.8³⁺Fe_0.3²⁺O₁₉ under different conditions were characterized by XRD, SEM, OM, and density measurement. The optimized conditions for synthesizing single-phase M-type barium hexaferrites with excessive Fe³⁺ and Fe²⁺ were obtained and were of 900℃, 30 min and 20 MPa and of 850℃,30 min and 10 MPa, respectively.A series of single-phase BaFe_12+xO_19+1.5xcompounds were synthesized under the optimized SPS condition of 900℃,30 min and 20 MPa. EPMA analyses show that the excessive Fe³⁺ were uniformly distributing in the BaM lattice. SEM observation shows that the grain size of BaFe_12+xO_19+1.5x compounds decreased with increasing x. VSM measurements indicated that both the specific saturation magnetizations and coercivities increased with increasing x. The variations of magnetic properties are attributed to the occupancy of excessive Fe³⁺ and the evolvement of microstructure.A series of single-phase BaFe_12-x³⁺Fe_1.5x²⁺O₁₉ compounds were synthesized under the optimized SPS condition of 850℃, 30 min and 10 MPa. SEM observation showed that the spherical grains grew up into platelet gradually with increasing x. VSM measurements indicated that the specific saturation magnetization almost remained constant as increasing x. The increase in the coercivity with increasing x in the range of 0-0.2 is linked to the enhancement of anisotropy field and pinning effect induced by the excessive Fe²⁺, and the decrease in the coercivity with increasing x in the range of 0.2-0.4 is attributed to the growth of grains.The molecular structure and electronic structure of BaFe_12+xO_19+1.5x were investigated with Raman, FTIR and XPS spectra. Raman and FTIR results show that the excessive Fe³⁺ occupied the octahedron interstices similar to the FeO₆ octahedron at the 12k site in the x range of 0-0.3 and the bipyramid interstices similar to the FeO₅ bipyramid at the 2b site for x=0.4, respectively. The occupations of excessive Fe³⁺ reasonablely explain the evolution of magnetic properties of BaFe_12+xO_19+1.5. XPS spectra revealed that the electronic structure of Fe and Ba remained invariable, while the binding energy of Ols increased with increasing x in the range of 0-0.4.